Neuronal nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain where they are involved in a variety of physiological processes, including cognition and development. The nAChRs are ligand-gated cationic channels, and different subtypes are known to be differentially permeable to Ca2+; the 7-containing nAChRs are generally considered to be the most permeable. Ca2+ can activate and regulate a variety of signal transduction cascades, and the influx of Ca2+ through these receptors may have implications for synaptic plasticity. To determine the Ca2+ permeability of the nAChRs in rat hippocampal interneurons in the slice, which contain diverse subtypes of 7- and non-7-containing nAChRs, we combined patch-clamp electrophysiology recordings with conventional fura-2 fluorescent imaging techniques. We estimated the relative Ca2+ permeability of the channels by determining the ratio of the increase in [Ca2+]i level ([Ca2+]i) in the soma to the integrated transmembrane current (charge, Q) induced by the activation of the nAChRs, and compared this ratio to the highly Ca2+ permeable NMDA subtype of glutamate receptor channel. In all cells tested, the [Ca2+]i/Q ratio was significantly larger (i.e. more than twice as big) for responses activated by NMDA than for 7-containing nAChRs in interneurons; the activation of the non-7 nAChRs did not produce any significant increase in [Ca2+]i. Interestingly, the Ca2+ permeability of native 7 nAChRs in PC12 cells was significantly larger than in hippocampal interneurons, and not significantly different from NMDA receptors. Therefore, the 7-containing nAChRs in rat hippocampal interneurons are significantly less permeable to Ca2+, not only than NMDA receptors, but also 7 nAChRs in PC12 cells.